A magnetic random access memory (MRAM) is a nonvolatile random access memory that utilizes a magnetoresistance effect element such as a magnetic tunnel junction (MTJ) element as a memory element. Since the MRAM is capable of achieving high-speed operation and can have infinite rewrite tolerance, research and development of the MRAM have been actively carried out to achieve commercialization in recent years. To further improve the versatility of the MRAM hereafter requires reduction in a write current and increase in a read-out signal. The reason is that the reduction in the write current not only lowers operation power consumption but also reduces costs due to reduction in a chip area, and the increase in the read-out signal shortens a read time which enables higher-speed operation.
As a write method for reducing the write current, the following spin transfer magnetization switching method has been proposed. Let us consider, for example, a memory element in which a first magnetic layer having reversible magnetization, a nonmagnetic layer and a second magnetic layer whose magnetization direction is fixed are stacked. According to the spin transfer magnetization switching method, a write current is supplied between the second magnetic layer and the first magnetic layer, and interaction between spin-polarized conduction electrons of the write current and local electrons in the first magnetic layer causes switching of the magnetization of the first magnetic layer. To directly supply the current in the memory element at the time of data writing is one of major differences as compared with a typical write method; a magnetic field application method (a method that supplies a write current to an interconnection arranged near the memory element and applies a resultant magnetic field to switch the magnetization of the first magnetic layer). Moreover, in the case of the spin transfer magnetization switching method, the magnetization switching (writing) is caused when current density excesses a certain threshold value. Since the current density is increased as a cell size is reduced, the write current can be made smaller with miniaturization. That is, scaling property of the write current is improved. Japanese Patent Publication JP-2007-142364 (hereinafter referred to as “Patent Document No. 1”) discloses material characteristics which can make the threshold current density for the magnetization switching equal to or less than a desired value. According to it, it is possible to reduce the threshold current density by using a perpendicular magnetization film as a magnetic layer and adjusting magnetic anisotropy energy density Ku and saturation magnetization Ms as appropriate.
Whereas, to increase a magnetoresistance ratio (MR ratio) of the magnetoresistance effect element is most effective for increasing the read-out signal. Development of an MTJ element that exhibits a high magnetoresistance ratio has been actively performed in recent years. Hayakawa et al., “Effect of high annealing temperature on giant tunnel magnetoresistance ratio of CoFeB/MgO/CoFeB magnetic tunnel junctions”, APPLIED PHYSICS LETTERS, Vol. 89, p. 232510, 2006 (hereinafter referred to as Non-patent Document No. 1) reports that a giant MR ratio (about 500% at room temperature) can be obtained in the Co—Fe—B/Mg—O/Co—Fe—B MTJ. The reasons that such a high MR ratio can be obtained in the Co—Fe—B/Mg—O/Co—Fe—B MTJ are considered to be as follows: (1) Co—Fe—B has high spin polarization, (2) (001)-oriented polycrystalline MgO that exhibits high spin filtering effect is formed by annealing Mg—O sandwiched between amorphous Co—Fe—B at high temperature.